Full color images, such as photographs, may contain an almost infinite range of color values. However, when printing a continuous tone image, such as a photograph, there is a limited number of colors available to recreate the image. As a result, in printing a continuous tone image, halftones are created. A common process for creating halftones is using a process called dithering, in which the density and pattern of color and/or black and white dots are varied to simulate different colors and different shades of gray. For a monochrome (black and white) image, areas of gray are created by varying the proportion of black and white dots or pixels, or by varying the dot size.
With respect to color, dithering approximates a color from a mixture of other colors when the required color is not available. Typically, the number of colors available is 256, such as seen in 8-bit GIF images. If a color does not correspond to one of the 256 available colors, the color is approximated from a mixture of two or more of the 256 available colors. The different colors can either be distributed randomly or regularly. Most images are dithered in a diffusion or randomized pattern to diminish any harsh transitions from one color to another color.
The dithering process differs for color and monochrome signals. Color signals are typically represented by multi-bit values, such as eight bits. In contrast, monochrome signals are typically represented as a single bit, meaning that a particular dot or pixel is either black or white. Because of the difference in bit size, halftone processors using a dithering process are not capable of outputting the color and monochrome signals simultaneously. As a result, it is not possible to output a color/monochrome pattern to one sheet of paper simultaneously.
Another halftone process that is commonly used is called error diffusion. Error diffusion works by thresholding each pixel value with a constant, and the difference between the pixel value and the constant is diffused to the neighboring pixels. More specifically, in a conventional error diffusion process, an algorithm calculates a quantization error for each pixel and feeds it forward to four unquantized pixels of an input image. The algorithm is equivalent to a feedback system that adds a weighted sum of four previous quantization errors to the current pixel before it is quantized. Since the weighting factors sum to one, the average value of the quantized image is locally equal to the true gray scale value, which yields images that are typically comparable or superior to most dithering techniques.
The output differs when applying error diffusion to monochrome data and to color data. In the conventional error diffusion process, the output for monochrome data is a single bit signifying that the pixel is either white or black. In contrast, the output for color data is a multi-bit value.
In addition to halftone processing, reproducing continuous tone images on displays and on printed paper typically uses gamma correction. Gamma correction controls the overall brightness of an image. Images which are not properly corrected can look either bleached out or too dark. Gamma correction of hardcopy devices, such as scanners, printers copier or multi-function peripherals, can be developed by analyzing a test pattern of a continuous tone image. As described above, however, it is not possible to output a color/monochrome pattern to one sheet of paper simultaneously. As a result, it is necessary to have separate test patterns for color and for monochrome.